The invention relates to diagnostic testing which involves detection of brain electric response to presentation of stimuli, e.g., auditory or electric.
It has long been known that time-varying spontaneous electrical potentials (SP) exist between different areas of a patient's scalp, and that a record thereof, called an electroencephalogram, or EEG for short, can be studied in an effort to relate them to the patient's brain activity. It is also known that when the patient is subjected to certain stimuli, a time-varying evoked potential (EP) tends to be superimosed on the normally present EEG voltages. For example, an auditory stimulus in the form of a click heard by a patient, tends to superimpose an oscillatory component on the SP. Similarly, an electric shock to the skin tends to superimpose an oscillatory component. The EP oscillation tends to be time-locked to the stimulus. Different stimuli tend to produce evoked potential signals with different anatomical distributions on the scalp and with different waveshapes.
The detection of evoked potentials and of the physiologically significant information therein is difficult both because the EP components tend to be inherently variable and because they have to be extracted from the extremely variable and often larger SP.
The superposition or averaging of a sequence of measured waveforms time-locked to repeated stimuli is used in an effort to emphasize the relatively stable EP components therein and de-emphasize the more variable SP components. Traditionally, the clinician relies on visual inspection of averaged waveforms in seeking to extract the relevant diagnostic features from a highly complex waveshape. The accuracy of the procedure depends to a great extent on the experience, skill and judgment of the particular clinician, and is immensely complicated by the high inherent variability of the relevant traces. Although this variability is largely due to ongoing physiological processes generating brain electrical activity, it is unrelated to the effects of the stimuli, and will be referred to as "noise" hereafter. It is highly desirable in certain cases simply to detect the presence of an evoked response in averaged waveforms. One simple example is when a need exists to known if the brain of a patient who is unable or unwilling to provide reliable subjective information shows an electrical response to sensory stimuli--for example in the case of a comatose or an anesthetized patient, a person with psychological impairments, an infant, or an uncooperative patient. It has been proposed in the past that the presence or absence of a significant brainstem evoked potential can be determined by finding the difference in spectral power between a patient's averaged waveforms in the absence and in the presence of an auditory stimulus, and comparing the difference with that recorded for a large population of previously tested "normal" subjects. In particular, it has been proposed to average a succession of 2,000 segments of a patient's brainwave signal in the absence of an auditory stimulus and another succession of 2,000 equally long segments time-locked to the presentation of an auditory stimulus, and to compare the difference in the spectral power of the two averages with the corresponding difference for a large population of normally functioning subjects known or believed to have responded in a certain way to a similar stimulus. See, for example, Laukli, E., et al., Early Auditory-Evoked Responses: Spectral Content, Audiology 20: 453-464 (1981). See, also, for background information, John, E. R., et al., Neurometrics, Science, Vol. 196, pp. 1393-1410, June 24, 1977 and U.S. Pat. Nos. 4,279,258; 4,201,224; 4,216,781; 4,188,956; 4,171,696; 3,901,215; 3,780,724; 3,705,297 and 3,696,808. As an example of a proposal for Fourier transform spectrum evaluations in connection with electroencephalography, see U.S. Pat. No. 3,725,690.
Despite the known efforts made in the prior art toward accurately detecting and interpreting EP, it is believed that much need remains for improvements which would make averaged waveforms evaluation more reliable and useful even in the absence of highly trained interpretation experts, and would reduce the incidence of false positive and false negative results. Accordingly, one object of the invention is to provide a method and a system for determining the presence of significant evoked potential in brainwaves, through an automated test which reduces the likelihood of subjective errors. Another is to carry out such test in a manner which tends to reduce the incidence of false positive and false negative results by departing from the known prior art and using primarily the brainwaves of a particular patient to determine how much of a difference between them in the absence and in the presence of a stimulus would be sufficient to indicate the presence of a significant evoked potential. Yet another is to determine the presence of a significant evoked potential in a manner which allows quantification of the likelihood that the results would be accurate for the particular patient being tested. Yet another is to end the test as soon as a sufficiently unambiguous determination has been made, and to end the test in any event if undue doubt still remains after a certain number of attempts.
In a specific and nonlimiting example of the invention, as applied to testing for the presence of significant evoked potential, a first set of averaged waveforms is derived from a patient in the absence of the selected stimulus and a second set of averaged waveforms is derived from the same patient in the presence of the stimulus. Respective spectral measures are found for the averaged waveforms when the stimulus is absent and a similar spectral measure is found for the corresponding averaged waveforms when the stimulus is present. The two sets of spectral measures are used to produce a test measure which reflects the difference between the mean values of the two sets of spectral measures in respect to their variance. This test measure is matched against an acceptance level and a rejection level, which in turn are functions of the number of averaged waveforms which go into finding the spectral power measures, and also reflect the desired degree of likelihood that the end result will be accurate for the particular patient under test. If the presence or absence of a significant evoked potential can be determined at this time sufficiently unambiguously, the end result is displayed and the test ends. Otherwise, the test goes on to process additional alterante sequences of averaged waveforms, one dervied in the presence and the other in the absence of the stimulus, until either (i) the presence or the absence of a significant evoked potential can be determined sufficiently unambiguously or (ii) a maximum allowable number of such sequences has been processed as described. In one example the stimulus is auditory; in another it is a skin electric shock (or any other stimulus useful for testing sensory pathways).